Immobile scattering centers

This photon beat method allows one to study the dynamics of the scattering centers in a frequency range (1 to 10 cycles) which is suitable for the overall motions of polymer chains. Furthermore, all the spurious signals caused by dust particles are easier to eliminate, since large particles are essentially immobile and contribute only to the elastic spectrum. 

In this case the screening arises because the solute particles are unable to follow the rapidly changing direction of the solvent flow. A similar situation prevails when ->0, for then the dumbbells are so massive that their motions are unaffected by the solvent flow. In both cases, the screening is identical in form to that produced by an array of immobile hydrodynamic scattering centers. 

Moreover, although placed inside of a lattice, the atoms are not immobile, yet having a vibration motion around their equilibrium positions due to the absolute temperature to which the crystal is considered. Accordingly, the X-ray scattering will be considered at the averaged position (in A) around the center of vibration of each atom through the so-called Debye-Waller factor. 

Here, the symbols and = n /p indicate the density and kinematic viscosity of the neat solvent (S) and q = q/q. A second system for which the response function is known is a solvent littered with randomly distributed immobile objects, each acting as a point center of hydrodynamic scattering with a friction coefficient characteristic of the strength of its (Stokes) interaction with the solvent. The response function specific to this second system is given by the formula... 

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